This invention relates to the noninvasive measurement of parameters such as blood pressure and oxygen saturation in man and animals, and more particularly to the optical noninvasive measurement of blood parameters with a body-member-illuminating inflatable cuff.
A number of noninvasive methods of measuring blood parameters are known. For example, blood pressure has been measured by the auscultatory method which uses a cuff and a stethoscope, and by the oscillometric method which only requires a cuff applied to a body member. The conventional oscillometric method relies on the small-amplitude pulsatilc pressure oscillations communicated to the cuff by the underlying artery in the body member during cuff deflation from above systolic pressure to zero pressure. Such arterial pressure oscillations cause corresponding oscillations in cuff pressure which can be amplified and used to identify systolic, mean and diastolic pressure. For example, it has been established by Posey et al. that the cuff pressure for maximal amplitude oscillations corresponds to mean arterial pressure. See Posey et al., xe2x80x9cThe Meaning of the Point of Maximum Oscillations in Cuff Pressure in the Direct Measurement of Blood Pressure,xe2x80x9d Part 1, Cardiovascular Res. Ctr. Bull. 8(1):15-25, 1969. See also Ramsey, xe2x80x9cNoninvasive Automatic Determination of Mean Arterial Pressure,xe2x80x9d Med. Biol. Eng. Comput. 17:17-18, 1979; and Geddes et al., xe2x80x9cCharacterization of the Oscillometric Method for Measuring Indirect Blood Pressure,xe2x80x9d Annals of Biomedical Engineering, Vol. 10, pp. 271-280, 1982. All such references are incorporated herein by reference.
Commercially available oscillometric devices are useful for some applications but are not particularly suited for use in neonatology and premature infants, for example, where very small amplitude pressure pulses are encountered in such small limbs, and in small animal veterinary medicine where animal limbs and tails, etc. are too small for conventional oscillometric devices.
The pulse oximeter is a known instrument for noninvasive measurement of oxygen saturation, as described in a review article by Geddes entitled xe2x80x9cHeritage of the Tissue-Bed Oximeter,xe2x80x9d published in IEEE Engineering in Medicine and Biology, 87-91, March/April 1997, said article being hereby incorporated by reference. However, conventional pulse oximeters do not include integral means for measuring blood pressure with the same sensors used for oximetry.
According to one aspect of the present invention, a transilluminating pressurizable cuff includes a hinged cuff housing having first and second semicylindrical sections and a hinge parallel to the longitudinal axis of the semicylindrical housing sections. One or more light sources are mounted on the first housing section, and a photodetector is mounted on the second housing section in a position to receive light transmitted from the light source(s) through a body member placed within the cuff housing. An optically transparent inflatable cuff is adapted to fit within the cuff housing and around the body member. The use of two circumferentially spaced light sources is preferred in order to minimize obstruction of the radiation beams by the bone.
One embodiment of the present invention provides an easily applied cuff for measurement of blood pressure and oxygen saturation in small body members, e.g., the limbs and tails of small animals and the limbs of infants, especially low birth-weight infants (below 8xc2xd pounds).
Another aspect of the invention is an optical oscillometric method which is useful for small body members. The conventional oscillometric method relies on small-amplitude oscillations, or pulses, in cuff pressure. Successful application of the conventional method requires the presence of detectable pressure pulses in the cuff. Such pulses are extremely small in small children, and even smaller in neonates and premature infants. In these subjects the conventional oscillometric method is unreliable, and it is also unreliable in small animals. Instead of detecting cuff pressure pulses, the oscillometric method according to the present invention detects optical pulses, such as from a photodetector optically linked to a light source in a pressurizable cuff. The new method allows reliable, efficient detection of pulses useful for obtaining systolic, mean and diastolic pressure in small body members. In fact, it has been found that, the smaller the member, the larger the optical pulsatile signal.